The present invention relates to primarily the preparation of automotive headlamp assemblies by bonding a polyarylate component (as described with more particularity hereinafter) to another such component or to a polycarbonate substrate.
Automotive headlamp assemblies in general consist of two parts: a headlamp housing in which the lamps are seated; and a transparent front cover. The seal bonding the transparent cover to the housing must meet rigid requirements which in general may be described as stability against weather and normal environmental use and conditions (other than collision) over the life of the vehicle.
In the evolution of material selection for such headlamp assemblies, it has now become common to manufacture the two components from polycarbonates. While the front cover will of course be transparent, the housing will be opaque with the interior having a reflective coating for increased illumination.
Prior to the present invention, such headlamp assemblies have in general been bonded or sealed by one of two methods: (1) a two-part thermally curable urethane adhesive; or (2) vibration welding. In the former, the two-part urethane composition is mixed and applied, the parts are assembled and the resulting assembly is then passed through an oven for a time and at a temperature to effect curing. In the vibration welding system, the assembled parts are subjected to vibration, e.g. sonic vibration, to melt and weld the surfaces together.
Both systems require capital expenditure for the necessary equipment. In addition, the vibration welding technique is limited by the geometric shape of the articles to be assembled. More importantly, neither system is sufficiently rapid to satisfy completely the manufacturing needs of an automotive assembly plant.
For these reasons, there is a great need in the automotive industry for a system for manufacturing headlamp assemblies more rapidly and with less capital expense and equipment.
In accordance with the present invention, the aforementioned objects are achieved in an elegant and highly efficacious manner by employing the photocurable compositions which will be described in detail hereinafter.
Photopolymerization, or radiation curing, as it is sometimes called, is in general well known. It has been referred to in the literature as "the quiet revolution". From a modest beginning in the early 1970's when it was employed primarily in printing, papermaking, and filling and coating flat wood stock, this "quiet revolution" in the technology has transformed it into a crowded art replete with patent literature. In recent years, this revolution has expanded the technology into the electronics industry, e.g. for photoresists and connectors, and for encapsulating components and sealing circuit boards, as well as into the fields of paints, fiber optics, jewelry, and dental and medical applications. Today, radiation-cured inks, coatings and adhesives are found in such diverse items as furniture, beverage cans, plastic containers, microchips and a host of other products.
Of the four major types of radiation curing known in the art, namely, ultraviolet (UV), electron beam (EB), infrared (IR) and visible light (VL) curing UV and EB have dominated the market, UV being most predominant. For example, in the early 1980's, UV curing was found in about 85% of the radiation-cured applications. EB, a relatively high capital-cost system, was the curing agent in nearly all the remaining applications, particularly for thicker, heavily pigmented coatings. IR curing, which was developed first, has been almost entirely supplanted by UV. Most recently, visible light curing has found application, mainly in such areas as dental restoration and tooth fillings.
In general, the purpose behind the development of radiation curing has been to eliminate the need for solvents and for ovens and the like for removing these solvents. Thus, radiation-cured inks, coatings and adhesives have afforded such advantages over their solvent-containing counterparts as being more economical, faster and safer. These advantages account primarily for the "quiet revolution" in the technology.
Typically, radiation-curable compositions, e.g. adhesive compositions, will include at least one suitable polymer or oligomer, a photoinitiator and a liquid monomer in which the various other ingredients are soluble, the monomer being cross-linkable with the polymer in the presence of the curing actinic radiation which, as mentioned, is UV light for most applications. In general, irrespective of the source of curing radiation employed, these liquid compositions may be characterized as being solvent-free and possessing an excellent shelf life, in that they may be stored in the absence of curing radiation for long periods of time.
The liquid monomers which have heretofore been suggested for use in radiation curable compositions include both monoand polyfunctional monomeric materials, the latter having more reactive sites to increase the crosslink density. Mixture of mono- and poly-functional monomers provides a useful way of varying the degree of hardness of the cured composition.
As examples of monomers heretofore employed, mention may be made of butanediol dimethacrylate, butoxyethyl methacrylate, butyl methacrylate, diethylaminoethyl methacrylate, diethylene glycol dimethacrylate, dimethyl-aminoethyl methacrylate, ethylene glycol dimethacrylate, 2-ethylhexyl methacrylate, ethoxyethyl methacrylate, glycidyl methacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, methyl methacrylate, neopentyl glycol dimethacrylate, polyethylene glycol dimethacrylate, tert.-butyl-aminoethyl methacrylate, triethylene glycol dimethacrylate, tetrahydrofufuryl methacrylate and trimethylolpropane trimethacrylate. Other monomers which may be used include acrylates such as butylene glycol diacrylate, n-butylacrylate, diethylaminoethyl acrylate, 2-ethylhexyl acrylate, ethoxyethyl acrylate, hexanediol diacrylate, polyethylene glycol diacrylate, phenoxyethyl acrylate, pentaerythritol triacrylate, trimethylolpropane triacrylate, triethylene glycol diacrylate, etc.; acrylamides such as N-isobutyloxymethyl acrylamide, N-methylolacrylamide, N,N,-dimethylacrylamide, N,N,-methylene bisacrylamide, etc.; allyl monomers such as allyl glycidyl ether, allyl methacrylate, diallyl phthalate, etc.; as well as various other monomers known in the art, including vinyl monomers, glycidyl ethers and the like.
The list of useful resins and oligomers includes: epoxy acrylate resins, oligoester acrylates, unsaturated polyesters, urethane acrylics, polymethylstyrene, styrene maleic resins, unsaturated polybutadiene hydroxyl terminated, tall oil and rosin derived resins, liquid polyamides, unsaturated alkyds, phenolics, vinyl esters, bisphenol type polyesters, halogenated polyesters, furan resins and the like.
The photoinitiators employed will vary according to the source of curing radiation. In typical UV systems, monomethyl ether of benzoin or a higher alkyl benzoin ether may be used as the activator along with or without a peroxide polymerization catalyst. Absorption of UV radiation causes the ether to decompose into free radicals which then initiate the polymerization reaction. On the other hand, visible light activated products usually employ a non-aromatic amine such as N,N-dimethylaminoethyl methacrylate in combination with a ketone or diketone such as camphorquinone, benzil, etc. The ketone or diketone absorbs radiation in the 400-500 nm range, producing an excited triplet state which, in conjunction with the amine, results in ion radicals.
While not intended to be fully comprehensive, the foregoing survey will serve to illustrate the extensive state of the art pertaining to photopolymerization and the long list of suitable monomers, resins and oligomers available for selection by the polymer chemist wishing to formulate a photopolymerizable composition.
As was mentioned previously, the patent literature is replete with references to various radiation-curable compositions, including combinations of polymers and monomers, which can be employed in such compositions. The following patents are illustrative of the state of the art.
U.S. Pat. No. 4,073,777 issued to O'Neill et al contains a detailed disclosure of unsaturated, water-dispersible polyester adhesives, films and textile finishes.
U.S. Pat. No. 4,082,710 issued to Vraneken et al dislcoses specified isocyanate-modified compounds consisting of the reaction products of an organic isocyanate with compounds with multiple acrylic radicals, which isocyanate-modified compounds can be polymerized in the presence of visible or UV light. They may be used either singly or mixed with other materials, such as inert non-copolymerizable polymers, reactive copolymerizable polymers, copolymerizable oligomers,inert plasticizers, inert organic solvents, copolymerizable olefinically-unsaturated monomer compounds and various adjuvants.
U.S. Pat. No. 4,181,752 issued to Martens et al relates to pressure-sensitive adhesives obtained by subjecting a solventless radiation-sensitive acrylate-containing polymerizable mass to radiation in the near UV region. Disclosure is made of procedures for the free radical copolymerization in the presence of UV of an acrylate, i.e. an acrylic acid ester of an alkanol and a monomer such as acrylic acid, methacrylic acid, acrylamide, acrylonitrile, methacrylonitrile, N-substituted acrylamides, hydroxy acrylates, N-vinyl pyrrolidone, maleic anhydride and itaconic acid.
U.S. Pat. No. 4,206,025 issued to Vraneken relates to specified acrylic polyesters that can be cured by UV or EB. These polyesters are described as meaning polymerizable organic compounds obtained by the polyesterification of dicarboxylic acid with a stoichiometric excess of OH groups of di-and polyhydric alcohols, the excess hydroxyl groups being then esterified with acrylic acid or one of its functional derivatives.
U.S. Pat. No. 4,530,746 issued to Azuma et al relates to photopolymerizable resin compositions comprising a monomer, e.g. a vinyl-end capped monomer, showing slight shrinkage upon polymerization. .Specifically, the disclosed compositions comprise: (a) the aforementioned monomer; (b) at least one epoxy-acrylate, 1,2-polybutadiene, polyester or organopolysiloxane resin having one or more acryloyloxy groups or methacryloyloxy groups on their molecular end or ends; and (d) a photosensitizer.
U.S. Pat. No. 4,533,446 issed to Conway et al discloses an anaerobic adhesive composition activatable by UV or visible radiation comprising: (a) an anaerobically polymerizable acrylate ester monomer, (b) a compound which decomposes upon exposure to ultraviolet or visible light to release a strong acid, (c) a peroxy free radical initiator, and (d) an activator of anaerobic polymerization which, in the presence of a strong acid, reacts with the peroxide initiator to catalyze polymerization of the monomer.
As mentioned earlier, visible light curable adhesives have to date found industrial application primarily in the dental arts, e.g. restoration and fillings. In general, such compositions employ one or more methacrylic monomers, one or more oligomers or polymers, and of course the photoinitiator system. As examples of illustrative patents pertaining thereto, mention may be made of U.S. Pat. Nos. 4,407,984; 4,439,380; 4,459,153; 4,525,256; 4,563,153; and 4,581,389.
The aforementioned copending application Ser. No. 947680 is directed to the aforementioned objectives for preparing headlamp assemblies utilizing polycarbonate substrates e.g. "Lexan" 141 (trademark of General Electric). In accordance with the invention described an claimed therein, these objectives are accomplished by employing a visible light curable composition consisting essentially of: one or more polyfunctional acrylic monomers; (2) a polyfunctional aliphatic urethane acrylate polymer; and (3) a catalyst or photoinitiator activatable by visible light.
The adhesive formulations of this prior application were satisfactory with the polycarbonate substrates such as "Lexan" 141 utilized in preparing the headlamp components. Recently, however, styling changes in the automotive industry have demanded headlamps significantly smaller in size, yet possessing comparable illumination capabilities. This design change has necessitated the use of higher output bulbs that operate at considerably greater temperatures. Unfortunately, the poor high temperature properties of polycarbonate have been shown to be unacceptable for the construction of headlamp housings in this application. As a result other materials with better high temperature resistance have been sought.
One of these materials, Lexan 4701, a polyester-polycarbonate copolymer manufactured by GE (also referred to in the art as a "polyarylate") has received increasing attention by the automotive industry as a high temperature substitute for polycarbonate. This change in substrate in turn resulted in failure of the adhesive formulations as described and claimed in the aforementioned pending application.
Accordingly the task of the present invention may be described as being to modify the forementioned adhesive formulations so that they may be employed with the new polyarylate substrates in preparing automotive headlamp assemblies.